The following listed references are expressly incorporated by reference herein. Throughout the specification, these references are referred to by citing to the numbers in the brackets [#].    [1] Control and System Integration of Micro- and Nano-scale Systems, Report from the National Science Foundation workshop Mar. 29-30, 2004, edited by Benjamin Shapiro, and available at www.isr.umd.edu/CMN-NSFwkshp/WkshpFullReport.pdf    [2] J. V. Clark, Electro Micro-Metrology, Ph.D. dissertation, Fall 2005, U. C. Berkeley.    [3] J. Marshal of the National Institute of Standards and Technology, 2005, August 2003, www.smalltimes.com/document_display.cfm?document id=6556    [4] J. Green and D. Krakauer, “New iMEMS Angular Rate-Sensing Gyroscope,” Analog Dialogue, 37-03, 2003, Analog Devices, Inc., available at www.analog.com/library/analogDialogue/archives/37-03/gyro.html
The present invention relates to an apparatus and method for improving the precision of nanoscale force-displacement measurements. Nanotechnology has great potential for being used to create new and improved medicines, materials, sensors, and devices through molecular-scale engineering [1]. The rate of such advancements depends on our ability to understand nanoscale phenomena. Therefore, with increasing interest in nanotechnology to improve the quality of our lives, there has been increasing interest in force-displacement measurements to improve our understanding of nanoscale phenomena.
Large uncertainties exist in conventional measurement tools which cause a bottleneck to technological advancements [1]. For instance, large uncertainties make it difficult to: 1) resolve important subtleties during the research phase; 2) verify or develop predictive models during the design phase; and 3) develop testing standards for the commerce phase. For example, most measurement tools only yield about two significant digits of accuracy. It is often quite difficult to match the mathematical physics of a simulation with an experiment when the experiment has such large relative error. Due to these large uncertainties, the industry has been slow to develop a consensus on the methods or the tools that are used to measure geometric properties, dynamic properties and material properties at the micro and nano-scale.
When engineering such micro and nano-scale systems, adequate computer aided design/engineering tools and metrology tools are needed. Metrology is the science of measurement. Scientists may discover a new nano-scale phenomenon and then try to exploit properties of the discovered phenomenon. The next step is understanding the discovered phenomenon. Scientists do this by developing a theory based on the physics that they understand at the time. Then, scientists try to match the theory with experiment. The next step is to build computer-efficient models of a phenomenon. Parameters are from metrology. The next step is to assemble the models developed into a system level simulation to try to predict the outcome of a new device. Often times, discovery which leads to an invention is made at this step. The next step is realization and finally, verification where metrology is used once again to see how well simulation has predicted reality. Metrology is used throughout this process which indicates its importance.
Two illustrative types of metrology are displacement and force. One of the most important nanoscale tools is the atomic force microscope (AFM), which is used to measure forces on the order of tens of piconewtons (similar to the force necessary to rupture DNA), used as a positioner, and used to measure displacements on the order of tenths of nanometers (similar to the size of atoms). However, precise calibration of the AFM has been difficult (˜1-15% precision [2]). The AFM is not sensitive enough to precisely characterize more subtle phenomena such as the van der Waals forces involved in protein folding (˜10−12N), the quantum vacuum forces involved in the Casimir effect (˜10−13N), or the Langevin forces involved in Brownian motion of bacterium (˜10−14N). Currently, subtle nanoscale phenomena are beyond precise verification and characterization, or possibly worse, beyond discovery.
When measuring force, a conventional mass balance can measure forces to a level of about a micro-Newton (10−6N), which is illustratively equivalent to solar radiation per m2 near earth. A conventional AFM can measure forces to a level of about a hundredth of a nano-Newton, illustratively the gravitational force between two 1 kg masses 1 m apart (10−11N). The improved force sensor of the present invention using the EMM calibration method can measure forces to a level of a pico-Newton, illustratively the light pressure of a 1 mW laser pointer or the forces due to protein folding, or even less in the range of 10−13N to 10−16N.
The atomic force microscope or AFM is the most popular nanoscale force detection tools used today. It consists of a cantilever, a very sharp probe tip and a laser beam which reflects off of the end of the cantilever onto a photodiode. Interaction forces between the tip and the target surface cause the cantilever to deflect. That small deflection is amplified by the reflecting laser light onto the photodiode which is used to measure that deflection. Knowing the deflection and knowing the stiffness of the cantilever allows one to measure force. The resolution of force of an AFM is on the order of 10 pico-Newtons. The AFM is also used as a positioner with a resolution of about a nano-meter.
Components of an AFM system include a cantilever, a laser diode, a mirror, a position sensitive photodetector, a feedback loop, piezoelectric scanner to move the sample, and a computer which performs the data acquisition, display and analysis.
There are many ways to calibrate the AFM cantilever. Three of the most popular are the thermal method, the added mass method and the unload resonance technique. There is no calibration standard and most conventional calibration methods yield about 1-15% uncertainty.
The illustrated apparatus and method of the present invention significantly increases the ability of scientists and engineers to sense and actuate at the nanoscale by several orders of magnitude. The present apparatus and method may benefit branches of biology, physics, chemistry, and engineering.
The present calibration system and method uses Electro Micro-Metrology (EMM) techniques, which are substantially more precise and practical than convention. Precise geometric, dynamic, and material properties at the micro/nanoscale can be extracted using electronic measurands. This is based on leveraging the sensitive electrical-mechanical coupling of microsystems to measure and characterize themselves.
In one illustrated embodiment, the present invention provides an on-chip, self-characterization method that differs from conventional metrology methods, which adapt macroscale tools and techniques to the micro/nanoscale, and which do not precisely determine the geometry and material properties due to process variation. EMM has several benefits over conventional methods as follows: 1) EMM does not rely on unconfirmed geometrical and material properties; 2) A multitude of properties can be extracted; 3) Uncertainties are much smaller and well-characterized, i.e. much more than two significant digits are attainable; 4) EMM measurements are performance-based and may lead to micro/nanoscale testing standards; 5) EMM's precision is reliable and repeatable; 6) EMM is nondestructive; 7) EMM is low cost; 8) The apparatus is small, lightweight, and portable; 9) It is automatable and amenable to industrial batch processing; 10) EMM is low-power; 11) EMM can be calibrated after packing, after a harsh environmental change, or after long-term dormancy; 12) The measurements are local; 13) Only a few test structures and a small amount of chip real estate are required; and 14) EMM is easier to use and measurements can be performed more quickly than convention.
Electro Micro Metrology (EMM) methods allow extraction of geometric, dynamic and material properties solely as functions of electrical measurands such as change of capacitance, change in voltage and/or change in frequency.
Geometric properties include:                1. Overetch        2. Sidewall angle        3. Gap spacing        4. Beam length        5. Area        6. Layer thickness        7. Beam width        8. Elongation        9. Comb finger offset        10. Etch hole        
Dynamic properties illustratively include:                1. Comb drive force        2. Displacement        3. System stiffness        4. Damping factor        5. Natural frequency        6. System mass        7. System damping        8. Velocity resonance        9. Displacement Amplitude        10. Quality factor        
Material properties illustratively include:                1. Base compliance        2. Webbing stiffness        3. Beam stiffness        4. System modulus        5. Shear modulus        6. Poisson's ratio        7. Strain        8. Stress        9. Material density        10. Material Young's Modulus        
In an illustrated embodiment of the present invention, a method is provided for improving precision of a nano-scale sensor. The method comprises fabricating a sensor on an integrated circuit chip, the sensor having at least one unknown property due to a fabrication process, determining the at least one unknown property of the sensor as a function of at least one electrical measurand associated with the sensor, precisely measuring the electrical measurand associated with the sensor, calculating the at least one unknown property of the sensor based upon the precisely measured electrical measurand, and using the at least one calculated property of the sensor to improve precision of the sensor.
In another illustrated embodiment of the present invention, a self-calibrating apparatus comprises a primary device fabricated on an integrated circuit chip. The primary device has at least one unknown property due to a fabrication process of the integrated circuit chip. The apparatus also comprises a test structure fabricated on the same integrated circuit chip as the primary device. The test structure has the same material properties as the primary device so that the test structure also has the same at least one unknown property as the primary device. The apparatus further comprises a electrical measurand sensor configured to measure an electrical measurand of the test structure, and a controller coupled to the primary device and electrical measurand sensor. The controller includes means for calculating the at least one unknown property of the test structure based on the measured electrical measurand. The controller uses the calculated at least one unknown property to calibrate the primary device.
In an exemplary embodiment, the electrical measurand sensor is fabricated on the same chip as the primary device and the test structure. The controller may also fabricated on the same chip as the primary device, the test structure, and the electrical measurand sensor. Illustratively, the at least one unknown property may comprises at least one of Young's modulus, density, stress, stain gradient, a geometrical error, viscosity, and stiffness.
In yet another illustrated embodiment of the present invention, an atomic force microscope having three degrees of freedom of movement comprises first and second anchors rigidly coupled to a substrate, first and second flexures coupled to the first and second anchors, respectively, a first plate coupled to the first flexure, a first drive actuator coupled to the first plate, and an electrode coupled to the first plate. The atomic force microscope further comprises a second plate coupled to the first plate by a third flexure, a third plate coupled to the first plate by at least one structures, the third plate also being coupled to the second anchor by the second flexure, a second drive actuator located between the second and third plates, and a probe tip coupled to the second plate. The first and second flexures and the first drive actuator provide a first degree of freedom, the electrode provides a second degree of freedom, and the third flexure and the second drive actuator provide a third degree of freedom.
The above-mentioned and other features of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of illustrated embodiments of the invention taken in conjunction with the accompanying drawings.